Should the dockerfile build the colcon workspace?

[ct2034]

A general thought: Why do we not build the workspace in the build? I mean in the dockerfile.
I do understand that during development that maybe didn't make a lot of sense because it was all WIP. But for the participants, I think it makes sense for the container to have the workspace build when you start it.
@sea-bass @dcconner What do you think?

[sea-bass]

We can switch to this once we have a more stable setup. However, it may be tricky because participants will want to rebuild the workspace, and it may become difficult to reuse the build that happened at image build time, to speed up local rebuilds.